random: convert GetRand{Micros,Millis} into randrange

There are only a few call sites of these throughout the codebase, so
move the functionality into FastRandomContext, and rewrite all call sites.

This requires the callers to explicit construct FastRandomContext objects,
which do add to the verbosity, but also make potentially apparent locations
where the code can be improved by reusing a FastRandomContext object (see
further commit).

src/net.cpp

@@ -3475,7 +3475,8 @@ std::vector<CAddress> CConnman::GetAddresses(CNode& requestor, size_t max_addres
         // nodes to be "terrible" (see IsTerrible()) if the timestamps are older than 30 days,
         // max. 24 hours of "penalty" due to cache shouldn't make any meaningful difference
         // in terms of the freshness of the response.
-        cache_entry.m_cache_entry_expiration = current_time + std::chrono::hours(21) + GetRandMicros(std::chrono::hours(6));
+        cache_entry.m_cache_entry_expiration = current_time +
+            21h + FastRandomContext().randrange<std::chrono::microseconds>(6h);
     }
     return cache_entry.m_addrs_response_cache;
 }